The properties of composites prepared with a matrix of biodegradable cellulose ester (cellulose acetate butyrate, CAB) and reinforced with regenerated cellulose lyocell fibers (lyocell/CAB) were studied and compared with short flax fiberreinforced composites (flax/CAB), used as reference. The effect of the lyocell fiber content on the composite properties was also investigated. Tensile properties, dimensional stability, fiber–matrix compatibility, and biodegradability were investigated by tensile testing, water absorption test, scanning electron microscopic analysis, and soil burial test, respectively. From the results, it was shown how the Young’s modulus of lyocell/CAB composites increased from 2 GPa for neat CAB to 4 GPa for a composite with a lyocell fiber content of 34.8% (v/v). Similar trend was obtained for flax/CAB biocomposites which showed higher modulus than lyocell/CAB composites, with values of 5 GPa for a flax/CAB composite with the same composition. Moreover, tensile strength of lyocell/CAB composites with fiber content higher than 16.7% (v/v) resulted in lower values than neat CAB, indicating a high probability of failure cracks on lyocell/CAB composite samples when increasing fiber composition. In addition, compared to neat CAB, elongation at break decreased for all the composites studied. It was also observed that increasing the fiber content, the water absorption of the composites increased compared to neat CAB matrix due to the hydrophilic nature of the lyocell and flax fibers. The biodegradation test showed, after 60 days of soil burial, about 10% and 25% of mass lost for 34.8% (v/v) of lyocell/CAB and flax/CAB composites, respectively.